"Roto peening", the use of rotating flaps having abrasive particles attached thereto, is gaining acceptance in the stress relieving, surface conditioning, and coating removal fields due to a variety of reasons. The process eliminates the use of "shot" to peen concrete and metal surfaces, and, where a coating is to be removed from such surfaces, no solvent need be used to loosen the coating Roto peening is thus an economical and environmentally sound procedure.
Flaps used in roto peening devices such as the wheel disclosed in Minnesota Mining and Manufacturing Company, St. Paul, Minn. (3M) product brochure no. 61-000-5490-4(1282)11, published December, 1988, typically include an elongate strap of material to which several metal (typically AISI 1006 or 1008 steel) peening particle supports are attached. As used herein, the term "peening particle support" means a structure including a metal base composition, the base having abrasive particles metallurgically fastened (for example, brazed) to an exposed surface thereof. Winter U.S. Pat. No. 3,834,200 describes such flaps in detail, as does the inventor's co-pending application Ser. No. 07/788,550, filed on even date herewith, both of which are incorporated herein by reference. Although the polyester polyurethane coated nylon fabric strap material and AISI 1008 steel peening particle supports of Winter have provided extended life in a variety of uses, in certain "high-intensity" peening operations, such as surface conditioning of concrete and removal of scale from steel, even longer flap life is desirable. As the useful life of a flap can be shortened either by strap or by peening particle support failure, the materials used in each are critical to performance of the flaps. This invention relates to improved peening particle supports which have provided longer life over previously known supports.
"Peening particle support failure", as used herein, means either the shank and head of a rivet used as support have been severed (rivet failure), or that the abrasive particles have been rendered ineffective. The latter can occur either by the particles falling off or by the particles actually being forced into the peening particle support material forming a flattened, less abrasive support. Each of these failure mechanisms is affected by the metallurgy and processing history of the peening particle support. Rivets made from AISI 1008 steel and other low carbon steels, while more easily cold formed, suffer from these failure mechanisms.
Peening particle supports, when used in the form of rivets, and flaps incorporating the rivets, are currently produced by a complicated series of steps. A rivet of AISI 1008 steel having dimples on its upper exposed surface is first produced by cold heading (forming) a piece of wire stock using a two stroke cold heading machine. The rivet is then stress relieved by heating to about 650.degree. C., after which the abrasive particles are brazed to the rivet dimples using a nickel (Ni) alloy brazing powder. The "brazed rivets" are then austenitized (heated above about 835.degree. C.), quenched and tempered (heating to about 700.degree. C.). The "tempered rivets" are then fastened to the flap using another cold forming step. Both the initial cold forming step and the final cold forming step are made easier by the low carbon content of 1008 steel.
Following the above procedures to make flaps, it was found that, although satisfactory for many applications, peening particle support failure limited flap life. This became especially true when stronger elongate strap materials made using linear polyurethane elastomer coated fabrics were used. Thus, an unmet need exists for a peening particle support which provides extended life of peening flaps in such high-intensity roto peening operations as mentioned above, but which is also easily formed into the desired shape using low power (about 3.times.10.sup.5 N force) cold heading machines.
U.S. Pat. No. 4,404,047 describes an electrical heating process for heat treating steels which includes a "temper straightening" step. Workpieces are preferably those having repeating cross-sections, such as bars, rods, tubes, and the like. While many grades of steel can be treated in this fashion, including AISI 1021 steel having 0.0029 weight percent boron (B), there is no suggestion of forming a peening particle support or rivet using the process or material.
U.S. Pat. No. 4,654,913 describes a method for producing captive washer wheel nuts wherein a variety of machining steps are performed on a nut-washer blank. Although this patent notes that any process may be used to form the blank, including cold forming of 10B21 steel (1021 steel having from 0.0005 to about 0.003 weight percent B), there is no further heat treatment of the blank which would suggest using 10B21 steel for roto peening.
U.S. Pat. Nos. 4,319,934, 4,322,256, and 4,322,247 describe a method of forming tools from alloy steels using severe cold forming, an alloy steel, and tools made therefrom, respectively. The steel preferably has from 0.28 to 0.33 weight percent C and 0.0005 to 0.0035 weight percent B, as well as other additives such as silicon (Si) and manganese (Mn), which apparently have the synergistic effect of allowing superior ductility and low work hardening for use in severe cold forming processes to provide products with high static and dynamic strength B is added to increase the homogeneity and hardenability of the steel. However, there is no suggestion or motivation to use such a steel in the formation of a rotary peen particle support. The material would be expected to be too hard (too high C content) to be cold formed using low power cold heading machines used to form rivet-type peening particle supports. This is evidenced by the 500 ton press used to cold form the tools.